PHYTOCHROME-MEDIATED LIGHT REGULATION OF PHYA-GUS AND PHYB-GUS TRANSGENES IN ARABIDOPSIS-THALIANA SEEDLINGS

Phytochrome wild-type gene-beta-glucuronidase (PHY-GUS) gene fusions w ere used in transgenic Arabidopsis to compare the activity levels and light regulation of the PHYA and PHYB promoters and to identify the ph otoreceptors mediating this regulation. In dark-grown seedlings, both promoters are 4-fold more active in shoots than in roots, but the PHYA promoter is nearly 20-fold more active than that of PHYB in both orga ns. In shoots, white light represses the activities of the PHYA and PH YB promoters 10- and 2-fold, respectively, whereas in roots light has no effect on the PHYA promoter but increases PHYB promoter activity 2- fold. Consequently, PHYA promoter activity remains higher than that of PHYB in light in both shoots (5-fold) and roots (11-fold). Experiment s with narrow-waveband light and photomorphogenic mutants suggest that no single photoreceptor is necessary for full white-light-directed PH YA repression in shoots, but that multiple, independent photoreceptor pathways are sufficient alone or in combination. In contrast, phytochr ome B appears both necessary and sufficient for a light-mediated decre ase in PHYB activity in shoots, and phytochrome A mediates a far-red-l ight-stimulated increase in PHYB promoter activity. Together, the data indicate that the PHYA and PHYB genes are regulated in divergent fash ion at the transcriptional level, both developmentally and by the spec tral distribution of the prevailing light, and that this regulation ma y be important to the photosensory function of the two photoreceptors.